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arXiv:2107.10086 · Europhysics Letters (EPL)

Dynamics in direct two-photon transition by frequency combs

Lin Dan, Hao Xu, Ping Guo, Jianye Zhao

Published Jan 1, 2022• 3 citations
arXivHTMLPDF

Abstract

Abstract Based on the proposed theoretical model of a three-level system, the optical Bloch equations including the direct two-photon transition (DTPT) process using the optical frequency comb (OFC) were derived and the population distribution of particles in the upper states varying with the velocity of the atoms was obtained. Comparing to the resonance two-photon transition process, that population was increased by a factor of 1.4 without the Doppler shift, which is consistent with our previous experimental results. Simultaneously, the relationships between momentum transfers, and atomic velocity and pulse number were analyzed. When applied to a multi-level system it was found that the population of particles in the excited states increased by a few percentages. The novel approach of DTPT using OFC improved the utilization of comb teeth and atoms, increased the momentum transfer path, reduced the reachable Doppler temperature limit, and encouraged us to use OFC to cool multiple elements simultaneously through the DTPT process. By analyzing the Doppler temperature of 133 Cs and 87 Rb in one dimension, it was found that this process can lower a temperature below 100 mK and generate dipolar molecules 133 Cs 87 Rb via photoassociation, which provides us with a new tool to create dipolar molecules and to investigate their complex rovibrational spectra in ultra-cold chemistry.

DOI
10.1209/0295-5075/ac47f4

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physics.atom-phCross-listedarXiv:2601.10575•Jan 16, 2026
Abstract Based on the proposed theoretical model of a three-level system, the optical Bloch equations including the direct two-photon transition (DTPT) process using the optical frequency comb (OFC) were derived and the population distribution of particles in the upper states varying with the velocity of the atoms was obtained. Comparing to the resonance two-photon transition process, that population was increased by a factor of 1.4 without the Doppler shift, which is consistent with our previous experimental results. Simultaneously, the relationships between momentum transfers, and atomic velocity and pulse number were analyzed. When applied to a multi-level system it was found that the population of particles in the excited states increased by a few percentages. The novel approach of DTPT using OFC improved the utilization of comb teeth and atoms, increased the momentum transfer path, reduced the reachable Doppler temperature limit, and encouraged us to use OFC to cool multiple elements simultaneously through the DTPT process. By analyzing the Doppler temperature of 133 Cs and 87 Rb in one dimension, it was found that this process can lower a temperature below 100 mK and generate dipolar molecules 133 Cs 87 Rb via photoassociation, which provides us with a new tool to create dipolar molecules and to investigate their complex rovibrational spectra in ultra-cold chemistry.
The performance of the cold atom clock based on coherent population trapping (CPT) improved when the temperature decreased. In order to obtain a lower temperature in the cold atom clock, we proposed a cooling scheme in this paper that employs direct two-photon transition using optical frequency combs (OFCs). Two trains of time-delayed pulses from opposite directions were utilized to interact with atoms. It was found that the temperature of the cold atoms reached the minimum if the pulse area was π and the time delay between the absorption pulse and the stimulated emission pulse was in the range from 0.7τ to τ. In this paper, it was confirmed that the proposed cooling process allowed for faster and more efficient momentum exchange between light and atoms, and the proposed cooling process could be applied to the atoms or molecules that could not be cooled to desired temperature through the single-photon cooling process. The 87Rb cooling, together with the CPT interrogating scheme using OFCs reduced the ratio value of linewidth/contrast, and the frequency stability of the cold atom clock hence improved by more than six times as per our calculation.
We discuss femtosecond Raman-type techniques to control molecular vibrations, which can be implemented for internal-state cooling from Feshbach states with the use of optical frequency combs (OFCs) with and without modulation. The technique makes use of multiple two-photon resonances induced by optical frequencies present in the comb. It provides us with a useful tool to study the details of molecular dynamics at ultracold temperatures. In our theoretical model we take into account decoherence in the form of spontaneous emission and collisional dephasing in order to ascertain an accurate model of the population transfer in the three-level system. We analyze the effects of odd and even chirps of the OFC in the form of sine and cosine functions on the population transfer. We compare the effects of these chirps to the results attained with the standard OFC to see if they increase the population transfer to the final deeply bound state in the presence of decoherence. We also analyze the inherent phase relation that takes place owing to collisional dephasing between molecules in each of the states. This ability to control the rovibrational states of a molecule with an OFC enables us to create deeply bound ultra-cold polar molecules from the Feshbach state.
We demonstrate the direct frequency comb spectroscopy of Rb 5S→5D two-photon transitions directly excited by an optical frequency comb (OFC) in 1.5 μm fiber communication bands. A chain of comb spectral manipulation and quantum coherence control is implemented to enhance the efficiency of second harmonic OFC generation and eliminate the Doppler-broadening background. The direct frequency comb spectroscopy with clearly resolved transition lines is obtained. Our scheme provides a potential approach to realize the OFC at ~1.5 μm with high stability and accuracy.
Abstract A single femtosecond optical frequency comb is implemented to induce two‐photon transitions between molecular vibrational levels to form ultracold molecules. The phase across an individual pulse in the pulse train is sinusoidally modulated with a carefully chosen modulation amplitude and frequency. Piecewise population transfer is fulfilled to the final state by each pulse in the applied pulse train providing a controlled population accumulation in the final, ultracold state. The dynamics of rovibrational cooling is discussed on an example of the KRb molecule. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010
A mode-locked femtosecond laser is used to physically map the laser frequency comb into the velocity comb of the excited Rb atoms at room temperature. Upon resonant excitation by discrete optical frequencies the velocity distribution of the excited Rb atoms shows comblike structure. Simultaneously, velocity-selective population transfer occurs between ground-state hyperfine levels. Both facts are observed by modified direct frequency comb spectroscopy and verified by a detailed density matrix treatment of the multilevel system in resonant field.
We demonstrate a Doppler thermometer based on direct optical frequency comb spectroscopy of an 85^{85}85Rb vapor with a chirped electro-optic frequency comb (EOFC). The direct EOFC Doppler thermometer is accurate to within its approximately 1 K statistical uncertainty. We experimentally compare direct EOFC spectroscopy with conventional Doppler spectroscopy using a single-frequency, step-scanned laser probe. Our results show that direct EOFC spectroscopy mitigates transit-induced optical pumping distortion of the atomic lineshape, which is the dominant systematic temperature shift in alkali atom Doppler thermometry. Optical Bloch equation simulations of conventional and direct EOFC Doppler spectroscopy confirm that EOFC spectroscopy can use higher optical power to reduce statistical noise without optical pumping distortion. Our results indicate that EOFC Doppler thermometry is a promising approach to realizing a primary thermometer with size and measurement rate sufficient for applications including pharmaceutical manufacturing and nuclear waste monitoring.